1. Field of the Invention:
The invention relates to the manufacture of polysuccinimide from aspartic acid. Polysuccinimide is obtained which has advantageously high molecular weight.
2. Discussion of the Background:
Polysuccinimide can be produced by cyclizing and polycondensing aspartic acid according to the following equation: ##STR1##
According to a method described by Neri, P., et al, 1973 J. Med. Chem., 16, 893, the polycondensation of aspartic acid is carried out by long heating at 180.degree. C. under reduced pressure, in the presence of 85% phosphoric acid. The initially pasty mass hardens to compacted pieces or, in the case of a polycondensation carried out by a rotary method, it hardens to a thin, coherent solid film. The coherent, solid polycondensation mass is refined to yield polysuccinimide with a molecular weight of about 100,000.+-.10,000 g/mol.
DD-PS 262,665 describes a method of manufacturing polyaspartimide (another systematic name for polysuccinimide, also known as polyanhydroaspartic acid). According to this patent, in a first step, aspartic acid is precondensed in the presence of polyphosphoric acid, under reduced pressure; and in a second step the polycondensate of molecular weight between 2,600 and 3,900, after removal of the polyphosphoric acid, is further condensed in the solid phase at 180.degree. C. under vacuum Depending on the duration of the subsequent condensation (4-12 hr), polyaspartimides (polysuccinimides) with molecular weights between 5,100 and 12,200 are obtained.
Polymer synthesis by polycondensation is always an equilibrium reaction, with a constantly decreasing reaction rate, and by the time the degree of polymerization reaches 1,000 the rate is practically zero ("Ullmann's Encyklopaedie der technischen Chemie", 4th Ed., Vol. 15, pp. 192-195). Results of this type were obtained in the work of Neri et al cited above.
The importance of polysuccinimide is particularly that it can be subjected to polymerization-type reactions to form other valuable polymers.
Thus, the .alpha.,.beta.-poly(2-hydroxyethyl)-DL-aspartamide described by Neri et al may be used as a plasma expander, and the polyaspartic acid derivatives obtained with amines of the formula H--N(A)B according to Ger. Pat. App. P 39 21 912.7 may be used in pharmaceutical and food preparations, where these polymers must be film-forming.
In the derivatization of polysuccinimide, for example, to form polyaspartamides by reaction of the polysuccinimide with amines, chain cleavage occurs, so that the new polymers have lower degrees of polymerization than the polysuccinimide starting material. Polymers with good film properties, however, have relatively high molecular weights. Film-forming polyaspartamides have molecular weights of at least 20,000-30,000. Derivatization to form usable polymer products is easier if the initial molecular weight of the polysuccinimide is high.